1. Field of the Invention
The present invention relates in general to a device for the homogenization of viscous fluids, and more especially to high-viscosity fluids, and also to a use of such a device. Particularly, but not exclusively, the present invention relates to a static mixer for use in a molding system for generating a substantially homogeneous flow of molding material, such as with the injection molding of preforms from polyethylene terephthalate (PET) and the like.
2. Background Information
A well-established application for static mixers is in conjunction with the extrusion and injection molding of thermoplastic materials, wherein the use of at least one static mixer in the melt stream provides for an improvement in the homogeneity of the melt properties. In particular, it is quite common to incorporate static mixers at the discharge of an injection assembly, in the melt channels of a mold runner system or within the nozzles adjacent the mold cavities. For example, U.S. Pat. Nos. 4,170,446 and 5,564,827 provide a mixing head in the machine nozzle that operates to counter the fact that: the plastic melt, upon leaving the screw, exhibits considerable temperature differences; when coloring agents and/or additives are added, their distribution can also be irregular; and the distribution of acetaldehyde (AA), a thermal degradation product of PET, can also be poorly distributed. U.S. Pat. Nos. 5,421,715, and 5,683,731 provide examples wherein static mixers are incorporated into the melt channels of a hot runner to provide melt property homogeneity, particularly with respect to melt temperature, AA distribution, and to reduce the disadvantageous effects of thermal degradation in the boundary layers. U.S. Pat. Nos. 5,941,637 and 6,089,468 provide examples of static mixers incorporated in the nozzle of a runner system to reduce the disadvantageous effects of the thermal degradation in the boundary layers and to eliminate weld lines.
The use of static mixers in multi-cavity preform molding systems, such as used in PET systems, is known to provide a more uniform AA distribution in the melt stream, which in turn provides for a more consistent AA level between preforms within the same molding shot. The amount of AA present in preforms, which are subsequently blown into beverage bottles, is of particular concern given that AA, even in small amounts, is known to impart an undesired taste to the beverage contained in the bottles. AA is a thermal degradation product of PET and is primarily the result of shear heating of the melt in the extruder, but also arises from boundary effects as the melt travels though the melt channels of the runner system to the molding cavities. European Patent Application 293 756 and U.S. Pat. No. 6,341,954 thoroughly discuss the problems associated with AA formation and distribution in a multi-cavity hot runner system.
The most common class of static mixers operates by dividing and recombining the fluid flow several times during its passage through the mixer, as exemplified in U.S. Pat. Nos. 5,564,827 and 6,394,644. The way in which this is done varies considerably, with a myriad of alternatives to both the basic configuration of the static mixer and the methods for their manufacture.
U.S. Pat. No. 5,564,827 describes a static mixer 10, as shown in FIG. 1, comprising webs 12 of bars 14 that are arranged in a plurality of layers along a longitudinal axis, the webs of adjacent layers crossing each other and being inclined relative to the longitudinal axis. The static mixer also includes a ring-shaped flange 16 situated at its middle. Further, the static mixer is formed as a monolithic structural member, essentially an integrally formed body, in an attempt to avoid weaknesses at the joints between the webs. However, the results from a stress distribution analysis conducted on such a mixer, at typical operating pressure of an injection molding system of between 1300 and 1700 bar (130 and 170 MPa), the static mixer made from wear-resistant cast non-ferrous alloy STELLITE 21, shows excessive localized stress at the connections 18 of the bars with the ring and connections 20 between bars. Indeed, the induced stress is calculated to exceed the strength of the material. In practice, the static mixer has proven to fail as predicted when used in the nozzle adapter of injection unit for an injection molding system producing preforms.
A variant to the previously mentioned static mixer, as shown in FIG. 1B, does away with the integrally formed ring, and is replaced with a separate supporting spacer 30 for supporting the mixing elements 32. The results from a stress distribution analysis conducted on such a mixer, and with the mixer element and supporting spacer made from STELLITE 21, again at typical injection molding pressure, predicts that the elimination of the ring has the effect of eliminating tangential (hoop) stress and outward radial pulling forces acting on bars. As a result, the stress level is generally lower. However, the design instead suffers from excessive stress in the vicinity of support areas 34 caused by the reaction forces exerted by the spacer. There is also excessive stress at the connections 20 between bars induced by rotation of the bars caused by collapsing of the structure under melt pressure. The stress exceeds the ultimate strength of material, and structural failure is again likely.
U.S. Pat. No. 6,394,644 describes a mixer element 40, as shown in FIG. 2, wherein multiple mixer elements may be stacked to form a static mixer. The mixer elements comprise a generally ring-shaped support 42 and a plurality of elongated crossbars 46 arranged in at least two separate intersecting oblique planes in the flow path to cause mixing of a fluid. The results from a stress distribution analysis conducted on such a mixer element, again at typical injection molding pressure and with the mixer element made from STELLITE 21, shows excessive stress at the connections 48 of the bars with the ring and at the connections 49 between the crossbars 46. The stress is understood to exceed the ultimate strength of material with structural failure of the mixer elements therefore likely.
Both the '827 and '644 static mixers are made of a metallic alloy by such methods as investment casting, and sinter injection molding, as generally described in U.S. Pat. No. 5,688,047. Such methods of manufacture are regarded generally as being technically challenging to execute, expensive (e.g. expensive patterns, costly die design and manufacturing). And in any event, without secondary processing techniques that incur additional expense, parts produced by such methods generally have mechanical properties that are less than identical parts machined from wrought or cast billets of the same material.
Another class of static mixers, worth mentioning for the sake of their configuration, operate by simply splitting the incoming melt stream into multiple paths that are then reoriented and discharged into a chamber prior to entering a second stage of splitting and reorientation, as exemplified in U.S. Pat. No. 3,583,678. However, the channels do not intersect and therefore mixing efficiency is low.
Therefore, the most basic problem with known static mixers is that they do not provide the desired combination of effective mixing, particularly in view of AA distribution in plastic melts of PET, with the required resiliency to indefinitely withstand the substantial and cyclic operational pressures of a plastic injection molding system. In practice, the failure of a static mixer in an injection molding system has proven to be catastrophic, with mixer remnants flowing throughout the mold runner system, requiring a complete refurbishment of the tool and lost production, both at a significant financial cost.
In addition, it is desired to have a static mixer that provides effective mixing and has the required resiliency to indefinitely withstand use in a plastic injection molding system. This is particularly true when considering the desire to provide a static mixer of an overall size that may be retrofitted into existing systems to replace known static mixers that have proven incapable of withstanding the operating conditions over long durations of time. Further, it is desired to be able to produce static mixers at a much lower cost than previously possible, along with other desirable attributes such as a low pressure-drop and minimal melt stagnation.